Reducing Steps and Hazardous Solvents

When chemists plan a synthesis for industry, making the right product is only part of the job. The process should also be efficient, safe and sustainable. Two ways to achieve this are to use as few steps as possible and to avoid hazardous solvents and starting materials.Why fewer steps? Some product is lost at every step, because yields are never 100%. The overall yield is found by multiplying the yields of the individual steps, so it falls quickly as a route gets longer. Each extra step also uses more reagents, energy, time and labour, and produces more waste.overall yield = yield₁ × yield₂ × yield₃ ...Worked example: a three-step route has yields of 80%, 75% and 90%. Overall yield = 0.80 × 0.75 × 0.90 = 0.54, so only 54% of the possible product is made. A two-step route to the same product with yields of 80% and 90% gives 0.80 × 0.90 = 0.72, which is 72%.Why avoid hazardous solvents? Many organic solvents are toxic, flammable or harmful to the environment, for example chlorinated solvents such as dichloromethane. They must be stored, recovered and disposed of safely, which adds cost and risk. Chemists therefore aim to design processes that do not require a solvent, or that use a safer one such as water, and that use non-hazardous starting materials.These choices go together with a high atom economy: a short route that uses safe materials and makes few by-products is cheaper and greener.